The invention pertains to the field of electronic device fabrication. More particularly, the invention pertains to processes of deposition of dielectric materials by physical vapor deposition.
Dielectrics are a class of materials characterized, in part, by their high resistance to the flow of electrical current. This characteristic introduces certain challenges to the formation of thin films using these materials, such as the ability to reliably produce a precise film thickness. Of particular interest is the deposition of a class of dielectrics known as piezo-electrics. Among applications of piezo-electric materials is the fabrication of bulk acoustic resonators, and, in such devices, the thickness of the piezo-electric material is of critical importance to device performance.
In practice, the piezo-electric material is often deposited via physical vapor deposition (PVD). PVD is a process wherein a target material is subjected to bombardment by electrically-excited ions in a vacuum chamber. This ion bombardment erodes the target and produces a vapor of the target material which ultimately coats all exposed surfaces, including that of the substrate, with a film of the target material. The electrical excitation is provided by an external power source which is most often configured to supply the deposition system with constant power for a fixed length of time. Under an assumption that the deposition rate is constant with applied power, the net deposited film thickness is thus predictable.
However, while the dielectric coating constitutes the desired deposition of the target material on the substrate, its accumulation on other system surfaces contributes to continuous fluctuations in the net system impedance seen by the power supply. These impedance variations automatically induce corrections in the power supply""s output to enable it to maintain the requested constant power level. These corrections directly affect the stability of the deposition rate, resulting in excursions from the expected constant rate and thus errors in the deposited film thickness. For deposition of very thin films, these errors may be inconsequential; however, for the relatively thicker films (greater than 1 micron) required in some applications, including bulk acoustic wave devices, these rate fluctuations accumulate to yield unacceptable errors in net film thickness. The total thickness of the piezo-electric material in these devices is often ten times the thickness of the same material in other types of devices. As a result, deposition times are extended, thus allowing for increased errors in total thickness. Other techniques for in situ monitoring thin-film depositions (mass oscillators or laser probe systems, for example) are inappropriate or difficult to adapt to this technology. Therefore, a need exists for an accurate and reproducible method to predict the thickness of such films.
The invention embodies a method and apparatus for controlling the thickness of a dielectric film formed by physical vapor deposition (PVD). The method compensates for the continuously varying electrical load conditions inherent in dielectric deposition via PVD. The method can be implemented through three different stages. Initially, the system power supply can be configured to operate in either constant current or constant voltage mode, herein referred to as constant supply parameter mode. Next, a gas composition which minimizes excursions in system impedance under these conditions is empirically determined. Finally, a test deposition can be performed using the constant parameter power supply mode and the gas mixture. This deposition is performed while tracking and summing the energy delivered to the system. The thickness of the deposited film is subsequently measured, and from these data a thickness-per-unit-energy relationship is determined. Depositions of predictable film thickness are then reproducibly performed under these established conditions. In practice, a given deposition is terminated at a value of total energy as determined by the established thickness per unit energy value and the required film thickness. The method is much more reliable than the current art technique of deposition at constant power for a fixed time.